Cool idea, enjoyed trying it!
Have few ideas to share:
1. After few level "+" and "-" abilities becoming too weak compared to "X" ability. What about making some benefits of using them? For example, if after using "+" result ends with 0 it is squared or cubed, or if after using "-" it ends with 5, boss is divided by 2.
2. You progress too fast comparing to the strength of the boss. What about increasing difficulty by adding some abilities like you cant us "-" numbers, or you can use only 2 digit numbers and so on
Good luck with your project!
Funnilly enough, I had the opposite experience! I got a combo of [2 of the same numbers = 2x damage], [the - operation squares the number used], and [all positive values deal damage instead of healing]... I never used multiply as it was SIGNIFICANTLY weaker! I was one-shotting level 50 Numbosses feelin just like a boss 😎